Sacrificial germanium layer for formation of a contact

ABSTRACT

A contact to a semiconductor substrate including a contact opening extending through an insulating layer to a doped active region of the semiconductor substrate. The contact opening can have a relatively high aspect ratio of 2:1 or greater. The contact further includes a refractory metal germanosilicide region at the bottom of the contact opening, a refractory metal germanide layer at the sidewalls of the contact opening, and an overlying refractory metal nitride layer. The refractory metals of the invention include at least tantalum, titanium, cobalt and mixtures thereof. The contact is metallized, preferably using tungsten or aluminum. The method of manufacturing the contact comprises etching the contact opening. A germane gas is used to clean native silicon dioxide from the bottom of the contact opening and to deposit a germanium layer thereon. A refractory metal layer is deposited over the germanium layer. After annealing in a nitrogen atmosphere at a temperature of about 600° C. or less, the contact opening is metallized with tungsten or aluminum.

This is a continuation-in-part of U.S. patent application Ser. No.08/816,165, filed Mar. 12, 1997, which is a divisional of Ser. No.08/503,385 now U.S. Pat. No. 5,644,166, filed Jul. 17, 1995 and issuedJul. 1, 1997, each of said applications being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to the formation of high aspect ratiosubmicron VLSI contacts. More specifically, the present invention isdirected to depositing a germanium layer into a contact opening usinggermane gas in order to remove native silicon dioxide from the contactopening. The germanium layer at the bottom of the contact opening isconsumed during annealing to form a low resistance contact.

2. The Relevant Technology

Modem integrated circuits are manufactured by an elaborate process inwhich a large number of electronic semiconductor devices are integrallyformed on a semiconductor substrate. In the context of this document,the term “semiconductor substrate” is defined to mean any constructioncomprising semiconductive material, including but not limited to bulksemiconductive material such as a semiconductive wafer, either alone orin assemblies comprising other materials thereon, and semiconductivematerial layers, either alone or in assemblies comprising othermaterials. The term “substrate” refers to any supporting structureincluding but not limited to the semiconductive substrates describedabove.

The movement toward progressive miniaturization of semiconductor deviceshas resulted in increasingly compact and efficient semiconductorstructures. This movement has been accompanied by an increase in thecomplexity and number of such structures aggregated on a singlesemiconductor integrated chip. As feature sizes are reduced, newproblems arise which must be solved in order to economically andreliably produce the semiconductor devices. The submicron features whichmust be reduced include, for instance, the width and spacing of metalconducting lines as well as the size of various geometric features ofactive semiconductor devices.

As an example, the requirement of submicron features in semiconductormanufacturing has necessitated the development of improved means ofmaking contact with the various structures. The smaller and more complexdevices are achieved, in part, by reducing device sizes and spacing andby reducing the junction depth of regions formed in the semiconductorsubstrate. Among the feature sizes which are reduced in size are thecontact openings through which electrical contact is made to activeregions in the semiconductor devices. As both the contact size andjunction depth are reduced, new device metallization processes arerequired to overcome the problems which have been encountered.

Historically, device interconnections have been made with aluminum oraluminum alloy metallization. Aluminum, however, presents problems withjunction spiking. Junction spiking results in the dissolution of siliconinto the aluminum metallization and aluminum into the silicon.Typically, when aluminum contacts with a silicon substrate directly, thealuminum eutectically alloys with the silicon substrate at temperatureslower than 450° C. When such a reaction occurs, silicon is dissolvedinto the aluminum electrode, and there is a tendency for silicon thusdissolved into the electrode to be precipitated at a boundary betweenthe electrode and the substrate as an epitaxial phase. This increasesthe resistivity across the contact. Furthermore, aluminum in theelectrode is diffused into the silicon substrate from the electrode andforms an alloy spike structure in the substrate.

The resulting alloy spike structure is a sharp, pointed region enrichedin aluminum. The alloy spikes can extend into the interior of thesubstrate from the boundary between the electrode and the substrate tocause unwanted short circuit conduction at the junction of thesemiconductor in the substrate, particularly when the junction is formedin an extremely shallow region of the substrate. When such an unwantedconduction occurs, the semiconductor device no longer operates properly.This problem is exacerbated with smaller device sizes, because the moreshallow junctions are easily shorted, and because the silicon availableto alloy with the aluminum metallization is only accessed through thesmall contact area, increasing the resultant depth of the spike.

Contact openings have also been metallized with chemical vapor depositedtungsten. This process has also proven problematic. The tungsten istypically deposited in an atmosphere of fluorine, which attacks thesilicon, creating “wormholes” into the active region. Wormholes canextend completely through the active region, thereby shorting it out andcausing the device to fail. Tungsten also presents a problem in that itdoes not adhere well directly to silicon.

3. Prior State of the Art

In order to eliminate the problems associated with the reaction betweenthe silicon substrate and the metallization material, prior artsolutions have typically used a diffusion barrier structure in which thereaction between the silicon substrate and the electrode is blocked by abarrier layer provided between the electrode and the substrate. Such abarrier layer prevents the diffusion of silicon and aluminum. It alsoprovides a surface to which the tungsten will adhere and which willprevent tungsten and fluorine from diffusing into the active region.

Prior art FIGS. 1 through 4 of the accompanying illustrations depict oneconventional method known in the art of forming contacts having adiffusion barrier. In FIG. 1, a contact opening 18 is etched through aninsulating layer 16 overlying an active region 14 on a substrate 12.Insulating layer 16 typically comprises a passivation layer ofintentionally-formed silicon dioxide in the form of borophosphosilicateglass (BPSG). Contact opening 18 provides access to active region 14 bywhich an electrical contact is made. Native silicon dioxide layer 20 isa thin layer which forms on the active region from exposure to ambient.As shown in FIG. 2, a titanium metal layer 22 is then sputtered overcontact opening 18 so that the exposed surface of active region 14 iscoated.

A high temperature anneal step is then conducted in an atmosphere ofpredominantly nitrogen gas (N₂). Native silicon dioxide layer 20 isdissolved and titanium metal layer 22 is allowed to react with activeregion 14 and change titanium metal layer 22 into a dual layer. As shownin FIG. 3, a titanium silicide (TiSi_(x)) layer 26 is formed by theanneal step, and provides a conductive interface at the surface ofactive region 14. A titanium nitride (TiN_(x)) layer 24 is also formed,and acts as a diffusion barrier to the interdiffusion of tungsten andsilicon or aluminum and silicon, as mentioned above. Under suchconditions, the lower portion of titanium metal layer 22 overlyingactive region 14, after dissolving native silicon dioxide layer 20,reacts with a portion of the silicon in active region 14 to formtitanium silicide layer 26. Concurrently, the upper portion of titaniummetal layer 22 reacts with the nitrogen gas of the atmosphere to formtitanium nitride layer 24.

The next step, shown in FIG. 4, is metallization. This is typicallyachieved by chemical vapor deposition (CVD) of tungsten, or by thedeposition of aluminum using any of the various known methods. Theseinclude aluminum reflow sputtering, and chemical vapor deposition. Inthe case of tungsten, the titanium nitride helps improve the adhesionbetween the walls of the opening and the tungsten metal. In the case ofboth tungsten and aluminum, the titanium nitride acts as a barrieragainst the diffusion of the metallization layer into the diffusionregion and vice-versa.

Spiking and wormholes can still occur, even with the use of a depositionbarrier, particularly when the diffusion barrier is too thin. Thisfrequently occurs at the corners of the contact opening, where it isdifficult to form a thick layer, particularly if the aspect ratio of thecontact is high. Contact opening 18 of FIG. 3 is filled by an aluminumlayer 32 in FIG. 4 which depicts the effects of spiking, with a spike 34extending through active region 14, the effect of which is to shortactive region 14 out.

The compound titanium nitride (TiN) is well suited to forming adiffusion barrier, as it is extremely hard, chemically inert, anexcellent conductor, and has a high melting point. It also makesexcellent contact with other conductive layers. Titanium nitride istypically formed by the reaction of sputtered titanium during annealingin nitrogen, or can be deposited directly on the substrate by reactivesputtering, evaporation, chemical vapor deposition and the like beforethe deposition of the metallization.

As device dimensions continue to shrink and the contact openings becomedeeper and narrower, contact walls become vertical and most of the metaldeposition techniques fail to provide the necessary step coverage tocreate adequate contact with the active area. Such narrow, high aspectratio contact openings can result in a partial or total failure to makesignificant contact with the active region. Accordingly, it becomesincreasingly difficult to produce the desired thickness of titanium atthe bottom of the contact opening.

FIG. 5 shows the dimensions used to calculate the aspect ratio, which isthe ratio of the height H to the width W. In order to introduce asufficiently thick titanium metal layer 22 using conventional sputteringtechniques and thereby create titanium nitride layer 24 such that isacts as an effective diffusion barrier, the aspect ratio of contactopening 18 is required to be kept relatively low, generally under 2:1.

The aspect ratios of contacts have been increased in the past bydepositing the titanium layer using a collimator to directly sputterdeposit plasma emanating from a target into the bottom of the contactopenings on a semiconductor substrate. The use of a collimator to directtitanium metal layer 22 in FIG. 2 to the bottom of contact opening 18prevents unwanted structures from forming on the walls of contactopening 18 and thereby plugging contact opening 18. A collimator havinga honeycomb structure has an aspect ratio corresponding to the thicknessof honeycomb structure divided by the diameter of the openings in thehoneycomb structure. In order to deposit the thick layers of titaniumneeded for this conventional method, the honeycomb structure used incollimator sputtering has been required to have a high aspect ratio,typically around 2.5:1. This slows down the manufacturing process andreduces throughput. Higher aspect ratios also require a high surfacearea of the collimator. A consequence of a high surface area is aconcomitant increase in particle contamination, and a reduced depositionratio on the wafer.

Other undesirable effects result from the conventional contact formingmethod. For instance, a high temperature of 800° C. or greater isrequired during the anneal step to properly form titanium silicide layer26 as shown in FIG. 3. In practice, high temperatures tend to cause lossto the titanium silicide layer and can cause the BPSG to crack and toreflow.

Another function of depositing a titanium layer in a contact opening isto remove native silicon dioxide (SiO₂) which forms whenever the siliconsubstrate is exposed to air. Typical native silicon dioxide layers havea thickness of about 20 Angstroms. Such a layer is shown at 20 in FIG.1. Native silicon dioxide layer 20 is highly insulative and can cause ahigh contact resistance so as to result in failure of the device.Titanium metal layer 22 of FIG. 2 serves to carry away oxygen, breakingdown native silicon dioxide layer 20. In the process, a portion oftitanium metal layer 22 is consumed. As a result, even more titaniummust be deposited in order to form an effective diffusion barrier.

Prior art methods employed plasma cleaning to remove the native silicondioxide from the bottom of the contact openings prior to depositingtitanium. These processes have proven unsatisfactory, as they are quiteexpensive, decrease throughput, and may require substantially higherrapid thermal processing (RTP) annealing temperatures. Furthermore,since native silicon dioxide grows in air, these methods do not preventthe reformation of native silicon dioxide in the contact openings oncethe methods are concluded.

For these reasons, there is a need in the art for an improved method ofcreating diffusion barriers in contacts that minimize the amount ofmaterial needed for effective diffusion barriers. This will in turnallow greater miniaturization of devices. Such a method would be moredesirable if it also had increased throughput, lowered costs, andincreased yields.

SUMMARY OF THE INVENTION

In accordance with the invention as embodied and described herein, thepresent invention comprises a submicron VLSI contact and a correspondingmethod for manufacturing the contact. The submicron VLSI contactcomprises a substrate having formed thereon an active region. Aninsulating layer such as silicon dioxide or BPSG overlies the activeregion. A contact opening is etched through the insulating layer toaccess the underlying active region. At the bottom of the contactopening is formed a refractory metal germanosilicide region. At thesides of the contact opening is a refractory metal germanide layer. Overthe refractory metal germanide layer and the refractory metalgermanosilicide region is a refractory metal nitride layer. Theremainder of the contact opening is filled with a metal such as tungstenor aluminum. The germanium used in forming the contact may be doped inorder to avoid depleting the active region.

The corresponding method of manufacturing the high aspect ratiosubmicron contact comprises the following steps. First, a doped activeregion is formed within the semiconductor substrate. The maximum depthof the doped active region defines a junction depth. An insulating layeris formed, typically by covering the active region with BPSG, reflowingthe BPSG, and planarizing it. Contact holes are then etched into theinsulating layer down to the active region, typically usingphotolithography and dry etch procedures. The contact opening is thenexposed to germane gas (GeH₄) at a temperature of between about 200° to600° C., at a pressure of 1 to 150 Torr, and for a period of time ofabout 60 seconds. This time may vary, but should be sufficient to removethe native silicon dioxide layer that has grown at the bottom of thecontact opening, and to deposit a germanium layer having a thicknessthat is preferably approximately the same as the thickness of arefractory metal layer that is to be subsequently formed. Next, therefractory metal layer is deposited over the germanium layer so as tohave a thickness less than about one-half the junction depth. Therefractory metal layer may be deposited with, for example, a sputteringprocess. Since the refractory metal layer may be much thinner than withconventional methods, the sputtering process may be completed with theuse of a collimator having a lower aspect ratio.

The next step is to anneal the contact opening in an atmosphere ofnitrogen gas (N₂). This is done at a lower temperature than theconventional method, with the preferred temperature being about 600° C.The anneal step causes a refractory metal germanosilicide region to format the bottom of the contact opening and a refractory metal germanidelayer to form at the sidewalls. An overlying refractory metal nitridelayer, which has been found to be an effective diffusion barrier, isformed over both the refractory metal germanosilicide region and therefractory metal germanide layer.

Since a much thinner refractory metal layer can be deposited, thecontact can have a higher aspect ratio. Aspect ratios greater than about2:1 are attainable. The improved diffusion barrier of refractory metalnitride effectively prohibits spiking and wormholes from forming in theactive region. Other advantages of the present invention include ahigher yield and a more stable BPSG layer due to the use of a lowertemperature anneal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained, a more particular descriptionof the invention briefly described above will be rendered by referenceto specific embodiments thereof which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be considered tobe limiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a cross-sectional elevation view showing the manner in which atypical contact opening is formed through an insulating layer to thesurface of a semiconductor substrate.

FIG. 2 is a cross-sectional elevation view illustrating the next step inthe conventional known method for producing a contact, comprisingdepositing a titanium layer into the contact opening.

FIG. 3 is a cross-sectional elevation view illustrating the next step inthe conventional known process for producing a contact, comprisingannealing the titanium layer in a nitrogen gas atmosphere to deposit anunderlying titanium silicide region and an overlying titanium nitridelayer.

FIG. 4 is a cross-sectional elevation view illustrating the next step inthe conventional known process for producing a contact, and comprisesmetallizing the contact opening. FIG. 4 also illustrates theconsequences of an insufficient contact barrier, which are shown asspikes penetrating through the active region.

FIG. 5 is a cross sectional elevational view showing the results of astep for producing a high aspect ratio submicron VLSI contact under thepresent invention, and comprises exposing the contact opening to germanegas to deposit a germanium layer over the bottom of the contact opening.FIG. 5 also shows the dimensions of the contact opening used incalculating the aspect ratio.

FIG. 6 is a cross-sectional elevation view illustrating the next step ofthe process of the present invention, comprising depositing a refractorymetal layer over the germanium layer.

FIG. 7 is a cross sectional elevation view illustrating the next step ofthe process of the present invention, which is annealing the contactopening in a nitrogen gas atmosphere to form a refractory metalgermanosilicide region at the bottom of the contact opening, arefractory metal germanide layer at the sidewalls of the contactopening, and an overlying refractory metal nitride layer.

FIG. 8 is a cross sectional elevation view showing the last step of theprocess, which comprises metallizing the contact opening with a metalsuch as tungsten or aluminum

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention comprises a high aspect ratio submicron VLSIcontact and a method for forming the high aspect ratio submicron VLSIcontact. The present invention utilizes a sacrificial CVD germaniumlayer in order to form a more intimate electrical contact, and a moreefficient diffusion barrier at the bottom of the contact. The method ofthe present invention is highly beneficial in the formation ofelectrical contacts to devices such as diodes, resistors, capacitors,transistors, and other semiconductor devices formed in high density onmicrochips. The method of the present invention, is shown by steps inFIGS. 1 and 5-8.

Shown in FIG. 1 is a substrate 12 as the surface of a semiconductorsubstrate. An active region is created on substrate 12 by doping aportion thereof. The resulting doped active region is seen at referencenumeral 14. The maximum depth of doped active region 14 defines ajunction depth as used herein. Specifically, the maximum depth of dopedactive region 14 is in the direction perpendicular to the plane definedby substrate 12. The junction depth is not a limitation on the presentinvention, which instead contemplates essentially any junction depththat may be used in the art. The junction depth is important to theextent that it correlates to a preferred thickness of a refractory metallayer that is to be subsequently formed during the method as disclosedherein.

Next, a protective insulating layer 16 is formed over doped activeregion 14. Insulating layer 16 preferably comprises BPSG in order toallow it to reflow at temperatures of about 900° C. or below. Insulatinglayer 16 is preferably reflowed and planarized to form a flat surface onsubstrate 12. In order to access the underlying doped active region 14,a contact opening 18 is etched through insulating layer 16 by a processof masking and etching, preferably dry etching, as is commonly known inthe art. Contact opening 18 extends through insulating layer 16 to dopedactive region 14 and has a bottom 15 and at least one sidewall 17.

In order to clean a native silicon dioxide layer 20 from bottom 15 ofcontact opening 18, and in order to form an effective diffusion barrierin preparation of metallizing contact opening 18, substrate 12 isexposed in a vacuum environment to germane gas (GeH₄). This ispreferably done using a low pressure chemical vapor deposition (LPCVD)technique. The process is preferably conducted with a pressure of about80 Torr, a temperature of about 500° C., a germane concentration ofabout 100%, and for a duration of about 60 seconds. The germane gaseffectively cleans native silicon dioxide layer 20 from bottom 15 ofcontact opening 18 by turning the silicon dioxide into a siliconsub-oxide (SiO_(x)) (X<2), which can be removed from the contact openingby sublimation in vacuum at a temperature of around 600° C. The cleaningof native silicon dioxide layer 20 from bottom 15 of contact opening 18allows for optimal electrical contact between the metallization layerand underlying doped active region 14. It also allows an overlyingrefractory metal layer to be as thin as possible.

The LPCVD process should be of sufficient duration to remove nativesilicon dioxide layer 20 and to also deposit a germanium layer 40 atbottom 15 of contact opening 18. In practice, for example, and not byway of limitation, the thickness of germanium layer 40 will typically bein a range from about 30 Angstroms to about 100 Angstroms. Morespecifically, however, the thickness of germanium layer 40 will beselected to be in a range less than to slightly greater than thethickness of a refractory metal layer 48 that is to be subsequentlyformed. Preferably, the thickness of germanium layer 40 and thethickness of refractory metal layer 48 are approximately the same. Thefactors that determine the actual dimensions of these thickness will bemore fully disclosed below.

As shown in FIG. 6, a refractory metal layer 48 is then formed overgermanium layer 40. Refractory metal layer 48 may be deposited bysputtering, CVD, or by other processes by which refractory metal isdeposited. As used herein “refractory metal” may be chromium, cobalt,molybdenum, platinum, tantalum, titanium, tungsten, zirconium, orcombinations thereof. It has been found that tantalum, titanium, cobalt,and a mixture of cobalt and titanium are particularly useful under thepresent invention.

Refractory metal layer 48 should be formed so as to have a thicknessless than about one-half the junction depth. Providing such a thicknesswill significantly reduce the likelihood that junction leakage willoccur in the completed contact. Limiting the thickness of refractorymetal layer 48 reduces the amount of silicon removed from the dopedactive area to form refractory metal germanosilicide during subsequentsteps of the method of the present invention. Since the refractory metalneed not react with the silicon dioxide as in the conventional method,refractory metal layer 48 may be much thinner than typically used,typically a reduction from about 150 Angstroms, as used in conventionalprocesses, to perhaps 50 Angstroms or less, depending on the junctiondepth.

Since less refractory metal need to be laid in bottom 15 of contactopening 18 than with the conventional process, the aspect ratio ofcontact opening 18 may be substantially increased. As a result, aspectratios above 2:1 are now attainable with the present invention. Thisincrease in aspect ratio in turn increases the number of devices thatmay be placed on a microchip, thereby aiding in the miniaturizationprocess.

Refractory metal layer 48 is preferably deposited using a honeycombstructured collimator sputtering technique. By allowing a thinnerrefractory metal layer 48, the aspect ratio of the holes in thehoneycomb structure of the collimator may be reduced. In conventionalprocesses, the aspect ratio of the collimator is about 2.5:1. Using thecurrent invention, this can be reduced to 2:1 or even as low as about1.5 to 1. This speeds up the process, and due to the reduced surfacearea of the collimator, results in lower particle contamination. Thiswill in turn result in a higher device yield.

During the LPCVD process, the germanium can be doped in situ, witheither N+ or P+ dopants, depending on whether the underlying junction isdoped with N+ or P+ dopants. This can be done by adding sources ofboron, phosphorus, arsenic or other dopants to the LPCVD procedure.Examples of dopants are phosphine (PH₃), used with a P+ active region,and diborane (B₂H₆), used with a N+ active region. This will preventgermanium layer 40 from reacting with and depleting the dopant of dopedactive region 14. Instead, the dopant concentration in doped activeregion 14 will be substantially consistent throughout the method of thepresent invention.

Next, contact opening 18 is annealed, the result of which is shown inFIG. 7. This is preferably done using rapid thermal processing (RTP) inan atmosphere of nitrogen gas (N₂) and for a time period of about 20 to60 seconds. The anneal step may be conducted at substantially lowertemperatures than with conventional techniques. For example,conventional techniques use a temperature of about 800° C. for theanneal, while the method of the present invention may use a temperatureof about 600° C. or less, with about 600° C. being preferred.

As a result of the anneal step, a refractory metal germanosilicide(RSi_(x)Ge_(y)) region 50, where R represents a refractory metal, isformed at bottom 15 of contact opening 18 and over doped active region14. A refractory metal germanide (RGe_(y)) layer 52 is also formed atsidewalls 17 of contact opening 18. The nitrogen gas also combines withrefractory metal layer 48 to form a refractory metal nitride (RN) layer54 above both refractory metal germanosilicide region 50 and refractorymetal germanide layer 52. Germanium layer 40 is sacrificially consumedin the process. The alloy will vary, but it is preferred that variable Xin (RSi_(x)Ge_(y)) have a value of about 1, that variable Y in(RSi_(x)Ge_(y)) have a value of about 1, and that variable Y in(RGe_(y)) have a value of about 2. As previously mentioned, refractorymetal, R, may be a combination of individual metals, for example,titanium and cobalt. In this case, RSi_(x)Ge_(y) could be expressed asTi_(w)Co_(1−w)Si_(x)Ge_(y)., where 0<w<1, and with variables X and Ypreferably each having a value of about 1.

Refractory metal germanosilicide can be formed at lower temperaturesthan refractory metal silicide (RSi_(x)), allowing a lower temperatureanneal. This has the additional benefits of stabilizing the contact,avoiding cracking or detrimental reflow effects of the BPSG insulatinglayer, and helping to maintain the size of the doped active region 14.

The final step, shown in FIG. 8, is metallization. In this step, ametallization material 56 is deposited in contact opening 18 and inphysical contact with refractory metal nitride layer 54 such thatcontact opening 18 is substantially filled. Metallization material 56 ispreferably tungsten formed in a CVD process or aluminum formed in areflow, sputter, or CVD process.

The resulting contact has high step coverage with strong adhesion, highelectrical conduction, and can be more easily miniaturized as a resultof the higher aspect ratio permitted. The process can also be conductedat lower temperatures and with higher throughput. Refractory metalnitride layer 54 acts as an effective diffusion barrier to resistpitting, spiking, and wormholes. The resulting microchip has betterreliability and a higher yield.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A method of forming a contact to a substrate, said methodcomprising: providing an active region in said substrate, wherein ajunction depth is defined as the maximum depth of said doped activeregion in a direction perpendicular to a plane defined by saidsubstrate; forming an insulating layer over said substrate; forming insaid insulating layer a contact opening extending to said active region,said contact opening having a bottom and at least one sidewall; exposinga silicon dioxide layer on said substrate over said active area togermane gas so as to react with and remove said silicon dioxide layerfrom said substrate, and to form a germanium layer at said bottom ofsaid contact opening and over said active region; and forming arefractory metal layer over said germanium layer, said refractory metallayer having a thickness that is less than about one-half of saidjunction depth.
 2. A method as recited in claim 1, wherein exposing saidsilicon dioxide layer on said substrate over said active area to germanegas is conducted by CVD of germanium.
 3. A method as recited in claim 1,wherein forming said refractory metal layer comprises deposition of arefractory metal material selected from the group consisting ofchromium, cobalt, platinum, tantalum, titanium, tungsten, zirconium, andcombinations thereof.
 4. A method as recited in claim 3, whereindeposition of said refractory metal material comprises sputterdeposition using a collimator having a honeycomb structure with anaspect ratio of less than about 2.5:1.
 5. A method as recited in claim1, wherein said germanium layer has a thickness that is about the sameas said thickness of said refractory metal layer.
 6. A method as recitedin claim 1, further comprising, after exposing said silicon dioxidelayer on said substrate over said active area to germane gas, annealingsaid substrate in a nitrogen gas at a temperature less than about 600°C. so as to form a refractory metal germanosilicide region at saidbottom of said contact opening, a refractory metal germanide layer atsaid at least one sidewall of said contact opening, and a refractorymetal nitride layer over said refractory metal germanosilicide regionand said refractory metal germanide layer.
 7. A method as recited inclaim 1, further comprising, while exposing said silicon dioxide layeron said substrate over said active area to germane gas, doping saidgermanium layer with a first dopant compatible with a second dopant thatis included in said active region such that a substantially consistentconcentration of said second dopant in said doped active region ismaintained.
 8. A method as recited in claim 1, wherein said contactopening has an aspect ratio greater than about 2:1.
 9. A method offorming a contact to a substrate, said method comprising: forming adoped active region in said substrate, wherein a junction depth isdefined as the maximum depth of said doped active region in a directionperpendicular to a plane defined by said substrate; forming aninsulating layer over said substrate; forming in said insulating layer acontact opening extending to said doped active region, said contactopening having a bottom and at least one sidewall; exposing at leastsaid bottom of said contact opening to germane gas to react with andclean a native silicon dioxide layer from said bottom of said contactopening and to form a germanium layer over said contact opening; forminga refractory metal layer on said germanium layer, said refractory metallayer having a thickness less than about one-half the junction depth;annealing said substrate in an atmosphere of nitrogen gas to form arefractory metal germanosilicide region at said bottom of said contactopening, a refractory metal germanide layer at said at least onesidewall of said contact, and a refractory metal nitride layer over saidrefractory metal germanosilicide region and said refractory metalgermanide layer; and forming a metallization material within saidcontact opening and in physical contact with said refractory metalnitride layer.
 10. A method as recited in claim 9, wherein forming saidrefractory metal layer comprises sputter deposition of titanium.
 11. Amethod as recited in claim 9, wherein forming said refractory metallayer comprises sputter deposition of tantalum.
 12. A method as recitedin claim 9, wherein forming said refractory metal layer comprisessputter deposition of cobalt.
 13. A method as recited in claim 9,wherein forming said refractory metal layer comprises sputter depositionof both titanium and cobalt.
 14. A method as recited in claim 9, whereinsaid annealing is conducted at a temperature less than about 600° C. 15.A method of forming a contact to a semiconductor substrate, said methodcomprising: forming a doped active region in said substrate, wherein ajunction depth is defined as the maximum depth of said doped activeregion in a direction perpendicular to a plane defined by saidsubstrate; forming an insulating layer over said substrate; forming insaid insulating layer a contact opening extending to said doped activeregion, said contact opening having a bottom and at least one sidewall,said contact opening further having an aspect ratio greater than about2:1; exposing at least said bottom of said contact opening to germanegas to react with and remove a native silicon dioxide layer from saidbottom of said contact opening and to form a germanium layer over saidcontact opening; forming on said germanium layer a refractory metallayer having a thickness less than about one-half the junction depth,said refractory metal layer being formed using a collimator having ahoneycomb structure with an aspect ratio less than about 2.5:1;annealing said substrate in an atmosphere of nitrogen gas at atemperature of less than about 600° C. to form a refractory metalgermanosilicide region at said bottom of said contact opening, arefractory metal germanide layer at said at least one sidewall of saidcontact opening, and a refractory metal nitride layer over saidrefractory metal germanosilicide region and said refractory metalgermanide layer; and forming a metallization material within saidcontact opening and in physical contact with said refractory metalnitride layer such that said contact opening is substantially filled.16. A method as recited in claim 15, wherein said germanium layer has athickness that is about the same as said thickness of said refractorymetal layer.
 17. A method as recited in claim 15, wherein exposing saidbottom of said contact opening to said germane gas comprisessimultaneously doping said germanium layer with a first dopantcompatible with a second dopant that is included in said doped activeregion such that a substantially consistent concentration of said seconddopant in said doped active region is maintained.